Using an injection signal to reduce the effect of capacitance changes in capacitive ECG recordings

Capacitive sensors allows to record the electrocardiogram (ECG) of patients in a more comfortable and unobtrusive manner than the conventional adhesive electrodes; they allow the recording of an ECG through insulating materials, e.g. textiles, and can thus be embedded in everyday objects like beds or car seats. Capacitive ECG sensing however still suffers from motion artifacts. More specifically, any distance change at the body-sensor interface (e.g. due to motion) causes a change of the coupling capacitance, which directly affects the recording. The motion artifact consists of a multiplicative and an additive artefact component. The latter dominates since it is proportional to the DC voltage across the coupling capacitor and often masks the features of interest of the ECG signal. Our on-going work consists in injecting a known signal through the recording system to track and compensate for the changes of the coupling capacitance and therefore reduce the motion artifact. A first method was proposed based-on a single-frequency injection signal. An identification scheme derived from a model of the capacitive sensing system was used to estimate the additive artefact and subtract it from the recording. An alternative method consists in reconstructing the ECG with a filter whose parameters are estimated based on a multi-frequency injection signal. The two proposed methods assume that the body-sensor interface is purely capacitive and that the motion causes a significant capacitance change. Some preliminary tests in simulation and lab environment showed that both methods lead to a significant improvement of the quality of the ECG recordings. The developed methods are currently further evaluated and compared in terms of performance and robustness for different recording scenarios. Motion artifacts due to capacitance changes are not the only interferences corrupting capacitive biopotential recordings. However, a proper understanding and reduction of these artifacts is a first step towards a robust capacitive system enabling unobtrusive ECG recordings on dressed patients at the hospital or at home.